Diode laser pumped, frequency doubled, solid-state lasers have been used for many years to produce laser beams in the visible wavelength range and are useful in a variety of applications. The efficiency with which the pumping laser beam energy is converted into the desired visible output laser wavelength is of critical importance in terms of the cost and size of the device that can be constructed.
International patent application number PCT/IB2004/001197 (Bright Solutions et al, publication number WO2004/095660) describes a laser apparatus for generating a visible laser beam using a minimum of optical components. The efficiency of the Bright Solutions design is limited by the need to use an active gain material with a linearly polarized emission, which restricts the available materials and therefore the choice of wavelength and output power levels.
International patent application number PCT/US2001/007166 (Melles Griot Inc, publication number WO2001/067562) also describes a laser apparatus for generating a laser beam using a minimum of optical components, however the efficiency of this design is limited by the intra-cavity depolarization losses caused by thermally induced birefringence within the active gain material and in addition it does not select the optimal polarization of the fundamental wavelength to allow efficient type 1 frequency doubling.
As reported in Optics Letters on Mar. 15, 2003, researchers at the University of Hamburg achieved a record efficiency for blue continuous wave laser power generation from a diode pumped solid-state laser by producing an output of 2.8 W of 473 nm light when pumped with 21 W of 808 nm radiation from a diode laser. In their design the efficiency is improved by placing a quarter-wave plate between the laser rod and the back mirror so that light that is depolarized by the laser rod's thermal birefringence is reversed on a second passage through the rod. In addition a Brewster Plate is placed between the mirrors to select the optimal linear polarization for frequency doubling. While the efficiency is improved by these means the additional components introduce unnecessary losses.